1. Field of the Invention
The present invention relates to a method for rolling tube blanks in a planetary skew rolling mill.
2. Description of the Related Art
In practice, tube blanks are loaded into planetary skew rolling mills discontinuously. Therefore, loading in a new tube blank inevitably leads to a downtime of the rolling mill which influences the overall performance of the rolling mill.
On the other hand, a continuous method of operating is known with cold pilger rolling. In this method, the blank to be cold pilger rolled is loaded without interrupting the rolling operation. The tube blank to be rolled is fed forward incrementally by driven feed carriages toward the rolling stand which moves backward and forward.
In addition, the tube blank is rotated incrementally during the forward feed by driven run-in and run-out chucks which are mounted upstream and downstream of the rolling stand.
German reference DE 33 04 002 C1 discloses a forward feed device for a cold pilger rolling mill having two feed carriages which are equipped with chucks. Each of these feed carriages is moved by two feed spindles arranged in parallel and on each side of the rolling line, via spindle nuts arranged in the feed carriage. This arrangement of the feed spindles permits a moment-free application of force to the feed carriages and continuous forward feeding of the blank, thereby allowing continuous rolling operation. German reference DE 29 22 941 C2 discloses a cold pilger rolling mill in which the first feed carriage is provided with a device for gripping the end of the blank during forward feeding. A device for pulling back the blank counter to the forward feed direction behind the mandrel stop assigned to the first speed-changing gear mechanism is assigned to the second feed carriage. The feed travel of the second feed carriage is dimensioned such that the front end of the blank being fed forward by the second feed carriage lies between the rolling stand and the driver device of the first feed carriage. This design and arrangement also permits continuous forward feed of the tube blanks to be rolled.
Cold pilger rolling requires both positive forward feed in the axial direction and positive rotation of the tube blank so that uniform rolling occurs over the circumference of the tube as a result of the conically profiled rollers in the rolling stand which moves backward and forward.
Skew rolling is different in that the roller arrangement of a skew rolling mill includes conical rollers arranged in a rotor which circulates about the tubular blank. The conical rollers arc rotated by the rotor in a rolling stand in the manner of a planet and thereby continuously draw the tubular blank into the rolling stand. Feed carriages are actually necessary only for rolling the blank head onto the roll stand. However, they may be optionally connected into the system during ongoing operation.
A further feature of skew rolling mills is that both the blank and the tube turn slowly as a result of torsion of the material during rolling. Both the direction of rotation and the angular speed are undeterminable before the rolling operation. The tube which is running out of the skew rolling mill may be required to be wound up behind the rolling stand. To allow for this, the tube must be prevented from rotating as it exits the rolling stand. This is achieved by varying the rotational speeds of the rotor and of the rollers with respect to one another. The rotor and the rollers are driven by separate motors in an arrangement referred to as a variable ratio planetary drive. A sensor, which detects rotation of the tube which is running out, is mounted on the runout side of the rolling mill. The rotation of the blank may be controlled such that the tube does not rotate only with an unacceptably high degree of expenditure on measurement and control. In this case, it would not be sufficient to change the rotational speed of the rotor and of the rollers with respect to one another but instead the position of the rollers would also have to be adapted to the tubular blank and to the tube. However, this can not be carried out with a running machine. For this reason, in most cases the tubular blank will rotate in an undetermined direction and with an undetermined angular speed.
The object of the present invention is to increase the performance of the planetary skew bevel gear rolling mill while taking into account the particular properties of this rolling method.
The object is achieved according to an embodiment of the present invention by continuously feeding the tube blanks which are to be rolled into a rolling gap of the roll stand end to end. A first tube blank is located in the rolling gap of the rolling stand. A second tubular blank directly following the first tubular blank is fed forward with a rotation corresponding to the rotation of the end of the first tubular blank caused by the torsion during rolling.
To this end, a device is provided for feeding tube blanks into a planetary skew rolling mill. A tubular blank may be rolled out by an internal tool which is fixedly held in position with respect to the rolling gap. The device includes a first holding device and a second holding device arranged at an axial distance from each other for holding a mandrel rod. Clamping jaws of the first and second holding devices may engage radially against the mandrel rod independently of one another. First and second forward feed devices for the tube blanks including clamping jaws are arranged for selectively engaging radially against the tube blanks independently of one another. Each of the first and second forward feed devices may be displaced with the tube blank in the forward feed direction or counter thereto. A device for pushing is arranged for pushing a new tubular blank onto the rear end of the mandrel rod which is held in the rolling position during the rolling of a preceding tubular blank. The movements of the holding devices and of the forward feed devices are capable of being matched to one another such that the rear end of the preceding tubular blank and the front end of the following tubular blank can be fed end to end to the rolling gap.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.